The problem with this question is that static friction and kinetic friction are not fundamental forces in any way-- they're purely phenomenological names used to explain observed behavior. "Static friction" is a term we use to describe the observed fact that it usually takes more force to set an object into motion than it takes to keep it moving once you've got it started.
So, with that in mind, ask yourself how you could measure the relative sizes of static and kinetic friction. If the coefficient of static friction is greater than the coefficient of kinetic friction, this is an easy thing to do: once you overcome the static friction, the frictional force drops. So, you pull on an object with a force sensor, and measure the maximum force required before it gets moving, then once it's in motion, the frictional force decreases, and you measure how much force you need to apply to maintain a constant velocity.
What would it mean to have kinetic friction be greater than static friction? Well, it would mean that the force required to keep an object in motion would be greater than the force required to start it in motion. Which would require the force to go up at the instant the object started moving. But that doesn't make any sense, experimentally-- what you would see in that case is just that the force would increase up to the level required to keep the object in motion, as if the coefficients of static and kinetic friction were exactly equal.
So, common sense tells us that the coefficient of static friction can never be less than the coefficient of kinetic friction. Having greater kinetic than static friction just doesn't make any sense in terms of the phenomena being described.
(As an aside, the static/kinetic coefficient model is actually pretty lousy. It works as a way to set up problems forcing students to deal with the vector nature of forces, and allows some simple qualitative explanations of observed phenomena, but if you have ever tried to devise a lab doing quantitative measurements of friction, it's a mess.)
The normal force does decrease with angle. This does not mean that the coefficient of friction changes:
We can, depending on the angle $\theta$ of the slope, split the gravitational force $F_g = mg$ acting upon a thing with mass $m$ resting on the slope into the normal force $F_n = mg \cos(\theta)$ and the force pointing down the slope, $F_s = mg\sin(\theta)$.
Now, the coefficient of friction is a property of materials, and does not change with the angle - but it is the case that the friction force will decrease since it is $F_k = \mu_kF_n$. The "greater propensity" of things to slide down steeper inclined slopes is due to the friction force decreasing, and due to the force pointing down the slope increasing with increasing angle.
Best Answer
It's so simple because it's only a first order approximation model to how friction actually works.
There are several other models, but to use them you usually need more parameters or other pieces of information about the system (for example, if there are fluid lubricants involved, the pattern of the surface, the materials involved, etc).
The model $F_f = \mu F_N$ is called Coulomb model of friction. It assumes 3 important laws:
1. Amonton's first law of friction
This law dates back to Leonardo da Vinci:
2. Amonton's second law of friction
Here is an example of experimental data showing the dependence of friction with normal force:
The slope gives the friction coefficient: $\mu = F_f/F_N$.
This also dates back to Leonardo da Vinci, who noticed that if the load of an object was doubled, its friction would also be doubled.
3. Coulomb's law of friction
This is only somewhat true for small changes in velocity. Some models account for this dependence:
a) Coulomb model (without static friction)
b) Coulomb model + viscosity (without static friction)
c) Coulomb model + viscosity
d) Coulomb model + viscosity + Stribeck effect
Limitations
Here is an example of experimental data showing the dependence of friction with velocity:
Here is an example showing non linearity with respect to the normal force:
The author comments on the graph above:
Here is another simple article about the limitations of the Coulomb model of friction.